The technical field of this application is vehicle chassis control with coordinated brake and steering control.
Braking or acceleration of a vehicle on a split coefficient surface, wherein one or more wheels on one side of the vehicle encounter a significantly larger coefficient of friction with the road surface than one or more on the other side of the vehicle, can produce a large difference in longitudinal forces that tend to produce an undesired yaw moment (rotation) of the vehicle. Although this tendency can be countered with a counter steering input, this is difficult even for experienced drivers, since it can happen quickly and without warning. Thus, the anti-lock braking systems (ABS) and companion traction control systems (TCS) of many vehicles are provided with means to detect a split coefficient condition and reduce the brake pressure apply rate on one or both wheels on the high coefficient side so as to slow down the effect of the yaw moment and allow the driver to recognize and provide a steering correction. But this compromise usually results in somewhat longer stopping distances in deceleration as some deceleration is given up for the increased vehicle stability.
The invention is a vehicle chassis control that reduces the need for compromise between deceleration/acceleration and stability in a split coefficient condition stores first and second calibrated values of a predetermined braking parameter. The first and second calibrated values of the predetermined braking parameter are calibrated to be used, respectively, when a steering correction is available and when it is not available. The control includes apparatus for detecting a split coefficient condition and, when it does and a braking signal is present, actuating braking apparatus for the wheel having the higher coefficient of friction with the first calibrated value of the predetermined braking parameter and simultaneously actuating the steering apparatus with a split coefficient steering correction to compensate for yaw induced by braking on the split coefficient road surface. If the split coefficient steering correction is not available, however, the braking apparatus is actuated for the wheel having the higher coefficient of friction with the second calibrated value of the predetermined braking parameter without simultaneously actuating the steering apparatus with the steering correction parameter. Thus, the system is always able to choose the best option for brake apply during a split coefficient condition by applying the steering correction with faster brake apply when possible but applying the brakes more slowly when steering correction is not possible. The split coefficient steering correction may have an open loop part derived from a difference between wheel slips of two wheels on opposite sides of the vehicle and/or a closed loop part derived from a yaw rate error, side slip or side slip rate error.